Propylene-based polymers, particularly homo-polypropylene (hPP) are well known in the art, and have long been used in the manufacture of fibers. Fabrics made from hPP, particularly nonwoven fabrics, exhibit high modulus but poor elasticity. These fabrics are commonly incorporated into multicomponent articles, e.g., diapers, wound dressings, feminine hygiene products and the like. While polyethylene-based elastomers, and the fibers and fabrics made from these polymers, exhibit low modulus and good elasticity, they also exhibit a tenacity, stickiness and hand feel which are generally considered to be unacceptable for commercial applications.
Tenacity is important because the manufacture of multicomponent articles typically involves multiple steps (e.g., rolling/unrolling, cutting, adhesion, etc.). Fibers with a high tensile strength are advantaged over fibers with a low tensile strength because the former will experience fewer line breaks (and thus greater productivity). Moreover, the end-use typically requires a level of tensile strength specific to the function of the component. Optimized fabrics have the minimum material consumption (basis weight) to achieve the minimum required tensile strength for the manufacture and end-use of the fiber, component (e.g., nonwoven fabric) and article.
Low modulus is one aspect of hand feel. Fabrics made from fibers with a low modulus will feel “softer”, all else equal, than fabrics made from fibers with a high modulus. A fabric comprised of lower modulus fibers will also exhibit lower flexural rigidity which translates to better drapability and better fit. In contrast, a fabric made from a higher modulus fiber, e.g., hPP, will feel harsher (stiffer) and will drape less well (e.g., it will have a poorer fit). Fabrics made from polyethylene-based elastomers feels very tacky and clammy to the skin.
Fiber elasticity is important because it translates to better comfort-fit as the article made from the fiber will be more body conforming. Diapers with elastic components will have less sagging in general as body size and shape and movement vary. With improved fit, the general well being of the user is improved through improved comfort, reduced leakage and a closer resemblance of the article to cotton underwear.
Accordingly, interest remains high in a polymer that exhibits good elasticity and tenacity and low modulus when in the form of a fiber, and articles made from such fibers.